Algae: Planktonic Groups

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EA-Plant Sciences

(R. Ross)

ALGAE: PLANKTONIC GROUPS

       

CONTENTS

	Page
Bacillariophyceae	1
Structure	1
Reproduction	4
Colony Formation	5
Arctic Habitats	5
Composition of Arctic Diatom Flora	9
Dinophyceae	10
Arctic Occurrence	14
Chrysophyceae	15
Arctic Occurrence	19
Xanthophyceae	20
Bibliography	23

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EA-Plant Sciences

(R. Ross)

       

ALGAE: PLANKTONIC GROUPS

       

BACILLARIOPHYCEAE

        The Bacillariophyceae or diatoms ( Bacillariophyta ) are a group of

microscopic unicellular plants belonging to the Algae. Like almost all

plants, they manufacture their own food from mineral salts, carbon dioxide,

and water, using energy from sunlight absorbed by their green pigments.

They occur in all waters, both fresh and salt, throughout the world and,

except in tropical oceans, are among the predominant forms of plant life

in regard to quantity. They are less affected by adverse conditions than

higher plants, and some species can survive exposure to a temperature of

-80°C. for long periods. Being unicellular, they can multiply rapidly as

soon as the water they live in thaws. They are particularly abundant in

arctic and Antarctic waters and form the starting point of the food chains

which lead through the smaller marine animals — copepods, medusa, pteropods,

etc. — to fishes, whales, and other marine mammals. The rich diatom popula–

tion is the principal reason for the abundant life in arctic seas.

       

Structure

        Individual diatoms are one-called and vary in size from about 0.005

millimeters long and about one-third as broad, to disks 2 millimeters in

diameter, or rod-shaped forms 5 millimeters long. In the majority, however,

the longest dimension is between 0.02 and 0.2 millimeters. Their chief

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distinguishing feature is that each cell has a boxlike siliceous exoskeleton

known as the frustules. This consists of two halves fitting over each other

like the top and bottom of a pillbox. Each half is normally made up of two

parts, the valve, which forms the top or bottom of the box, and the girdle,

which is the side. Although there is usually only one girdle attached to

each valve, in some species the valves are separated by a number of complete

or partial loops. Other species have perforate septa across the interior of

the frustule in a plane parallel to the valve. There may be only two of

these, one attached to each valve, or many which fit into each other.

        The frustules, especially its valves, is normally covered with regularly

disposed fine markings in the form of dots, bars, or a honeycomb. These are

cavities in the silica which are usually open on the inner side and closed

by a fine membrane on the other side. This membrane itself often shows

fine markings; whether or not it is perforate is still undecided in spite

of investigations with the electron microscope.

        There is great variety in the shape of the frustule and in the dispose–

tion and nature of the markings, and a full survey of the variations is

beyond the scope of this account. Reference should be made to the works

quoted in the bibliography for more detailed information than is given

here. many diatoms have circular valves, flat, convex, or undulating, and

have their markings arranged in a radial or concentric pattern. These,

together with a number of elliptical, triangular, or polygonal forms with

processes at their corners, comprise the order Centrales. In this order

also is included the suborder Soleniineae whose long cylindrical frustules

are usually very feebly silicified and have a complex connective zone

consisting of many incomplete hoops; their valves are small and offen

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markedly excentric. They are all planktonic and the cells unite to form

long chains, as they also do in another planktonic group, the family

Chaetopceraceae, which are illiptical or circular forms with long awnlike

processes.

        In the order Pennales, which comprises the remainder of the diatoms,

the markings of the valve are disposed about a longitudinal line and are

thus more or less bilaterally symmetrical. This order, which is numerically

larger than the Centrales, exhibits an even greater variety of shape and

structure. The longitudinal axis may be straight, arcuate, biarcuate, or

sigmoid, more or less median or markedly excentric, and is often raised on

a keel. The outline of the valve varies from the linear to the arbicular

and is sometimes cuneate.

        An organ known as the raphe occurs in some of the suborders of the

Pennales. This is a cleft, often oblique or folded, through the silica of

the valve, and normally lies in the longitudinal axis. It usually occurs in

both valves of the frustule, but in one family the Achnanthaceae, it is found on

only one valve. In two large families, the Bacillariaceae and the Surirellaceae,

the raphe is on a keel and is of a rather different structure. It consists of

a canal opening to the interior of the frustule by a series of pores and to

the exterior by a fine oblique slit. Only those diatoms which possess a

raphe can move, a fact which provides strong evidence for the view that their

movement, a comparatively slow creeping motion, is due to the streaming of

protoplasm along the raphe. Other theories, most notably the presence of

cilia, are still put forward but seem less well founded.

        The cell content is much less variable than the frustules. On the

inside of these is a lining of protoplasm which encloses one or two large

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vacuoles. The single nucleus occurs either in the bridge of protoplasm

separating the two vacuoles or else in that lining the frustule. Each cell

contains one to many olive-green bodies, the chromatophores. In [ ?] general,

the Pennales posses large flat chromatophores, usually two in number, while

the Centrales normally have numerous, small, lens-shaped ones. The olive–

green color is due to the yellow pigments being present in greater propor–

tion to the green than is the case in higher plants. The food reserves are

normally stored as oil ad a few oil globules are usually present in the cell.

       

Reproduction

        This is normally by binary fission. When the cell divides, two nes

valves are formed in the old frustule before the two halves of the old girdle

separate. These new valves, with their girdles, form the inner halves of the

new frustules and, since they cannot grow after they are formed, one daughter

cell is the same size as the parent while the other is slightly smalle d r .

Growth of the cell between divisions takes place by the two valves moving

farther apart. Repeated divisions accordingly reduce the average size of

the individuals in the population and if may fall to one-quarter of its

original figure.

        The original size is restored by a different type of reproduction, known

as auxospore formation, in which the cell contents emerge from the frustule

and new large-size individuals are formed. This is a sexual form of repro–

duction. Typically two individuals come together and, after a reduction

division, one, two, or four gametes are formed in each cell. These gametes

fuse in pairs, one from each cell, and the resulting zygotes grow rapidly

and form the auxospores, which lay down new, larger frustules about themselves.

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Apogamy, in which after the reduction division two nuclei from the same

cell fuse to form the zygote, occurs in some Pennales and is the rule in

the Centrales.

        Some planktonic species of diatoms form resting spores. The cell

contents contract away from the valves at the onset of unfavorable conditions

and lay down a thick-walled siliceous endocyst around themselves. This does

not resemble the normal frustule of the species; it has no fine markings and

usually bears spines. These resting spores survive the unfavorable period

and germinate, producing cells with normal valves, when favorable conditions

return.

       

Colony Formation

        Most diatoms live free and solitary lives but some attach themselves

to the substratum by secreting a gelatinous pad or stalk. In some such

forms this attachment forks when cells divide, and a colony on a much-branched

stipe results. Some raphe-bearing species form long, branched, gelatinous

tubes in which the frustules live. A more frequent type of colony is a

filament formed by the failure of the daughter cells to separate after cell

division. These are sometimes attached to a substratum by their terminal

frustule, or they may occur free. Many planktonic species form such chains;

others form colonies consisting of a number of cells embedded in a common

mucilaginous envelope.

       

Arctic Habitats

        In both fresh waters and the sea, there are two ecological groups of

diatoms - those which live on substratum, and the free-floating plankton.

In fresh waters, where their growth begins again each summer as the ice and

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and snow melt, diatoms form an olive-green to gray-green flocculent layer on

the bottom and on submerged plants, etc. in sunlight, photosynthesis is

often so active that bubbles of free oxygen are formed which, becoming en–

tangled in this layer, carry lumps of it to the surface. All except the

smallest bodies of water have their planktonic diatom flora, but little is

known of the plankton of any arctic lakes. The growth of diatoms, both

planktonic and bottom-living, is limited to depths shallow enough to permit

the penetration of enough light for photosynthesis. This limit depends on

the clearness of the water and varies from a few feet to more than fifty.

        Diatoms not only occur in practically all permanent bodies of water but

also in puddles, in which they grow rapidly, and in any damp places, such as

tufts of moss where they are found in quantity. Soils in temperate regions

usually have a considerable population of diatoms, mostly very small forms.

The presence of a well-developed diatom flora in Icelandic soils indicates the

strong probability that one will be found also in the Arctic.

        In general, arctic freshwater diatoms are smaller and have finer markings

than those of more temperate waters. Individuals from arctic localities are

not smaller a than those of the same species from farther south, but the

smaller and more finely marked species tend to extend farther north than the

larger and coarser ones. This appears to be a temperature effect, since

shallow waters well warmed by the sun contain larger and coarser forms than

are found in colde d r waters in the same district. However, the chemical

content of the water, in particular the pH and salinity, is often the

principal factor determining the diatom flora, diatoms being scarce in

acid waters poor in mineral salts, and frequent in waters of higher pH and

salt content.

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        Marine . Marine diatoms are plentiful along those arctic shores which

are free from ice in the summer months, growing on the larger algae, on

mollusk shells, stones, etc., and living free on the bottom in shallow

water, on the shore between tidemarks, and in rock pools — as in temperate

waters. In the Arctic, in addition, the ice floes support a large diatom

population containing many species not found elsewhere.

        Diatoms begin to grow on the ice about March. They grow on the

undersurfaces and sides of the floes, particularly on the ice foot, and in

pools in the surface of the floes. There they form small, round patches and,

since these, being dark-colored, absorb more solar energy than the surrounding

ice, they raise the temperature locally and melt the ice on which they are

growing. In this way the ice becomes pitted with holes giving rise to the

condition known as “rotten ice” by the early whalers. Among diatoms from

ponds on the ice, some freshwater species, and also the spores of marine

planktonic species, are often found. The spores are formed in the autumn

and incorporated in the ice as it freezes. The freshwater forms are not

found in large numbers and have only been seen in acid-cleaned preparations.

It is accordingly not known whether they were living on the ice, and indeed

it has been generally assumed that they were dead frustules, either blown

there or present because the ice had formed in a river estuary. The water

in the pools is often almost fresh, however, and the frequency of their

occurrence suggests that they may be able to maintain themselves there for

a time.

        In those parts of the Arctic Sea which are not ice-free during the

summer, there is little true phytoplankton. However, in the leads between

the floes, globular aggregations of littoral diatoms, often as large as a

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man’s fist, are found. These lumps, often present in large numbers, contain

species of other algal groups as well as [ ?] most of the diatoms found on

the neighboring ice, and they apparently originate from the film of diatoms

on the ice. They congregate at the interface between the salt and fresher

water which is formed one or two feet below the surface as the ice melts.

The sudden change of salinity encountered there result in the death of their

outer layers which become bleached in the later part of the summer.

        With the onset of winter and littoral diatoms of the ice fields and most

of those growing along the coasts become embedded in the newly formed ice.

All the species are able to survive this, although the majority of individuals

succumb. The survivors produce the next season’s crop when light and warmth

return in the spring.

        The planktonic diatoms fall into two groups. There are the truly oceanic

types which maintain themselves in open water throughout the year. Most of

these are discoid forms or species of the genus Chaetoceros . The Soleniineae,

which are prominent in the plankton of most seas, are comparatively rare in

arctic waters, although a few species are carried by Atlantic water into the

Greenland and Barents seas. The other group are the neritic types, which are

incapable of passing the whole year in the plankton and are only found in

coastal waters or near the ice fields. In some it is only the resting spores

which are not planktonic, but others, e.g., Melosire arctica , probably the

commonest arctic diatom, can flourish either on the ice or in the plankton.

A fuller account of the arctic phytoplankton will be found in the article

“Phytoplankton.”

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Composition of Arctic Diatom Flora

        The freshwater diatom flora of the Arctic contains few species

peculiar to the region but consists largely of forms also common in

temperate lands. Many found in warmer climates, however, are unable to

withstand arctic conditions and hence the arctic diatom flora is less varied

than that of countries farther south. It is nevertheless quite extensive,

and more than 500 species of freshwater diatoms have been recorded from north

of the Arctic Circle. The freshwater diatom flora of Ellesmere Island, our

present knowledge of which is based on only six gatherings, illustrates

these points. From these 72 species are known, of which 14 are confined

to the Arctic and Subarctic and 7 more elsewhere only occur in alpine habitats.

The remaining 70% of the freshwater diatoms from this island, which lies north

of latitude 75° N., are temperate species. Most diatoms found only in the

Arctic have a circumpolar distribution. Although many species have wide

ranges, the freshwater species peculiar to the Arctic [ ?] have not been found in

the Antarctic.

        The marine diatoms present a different picture. The location and move–

ment of the principal water messes control the distribution of planktonic

forms; hence the plankton characteristic of the cold arctic water is not

normally found outside high latitudes, except when carried southward by

such surface currents as the Labrador Current. Similarly the plankton diatoms

of temperate waters are carried into the Greenland and Barents seas by the

Gulf Stream. Many arctic marine plankton species are also found in the

Antarctic.

        While the diatoms found along the coasts of the Arctic Sea include many

temperate species, most of those found on the ice floes do not occur outside

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the Arctic. Again, while some of these forms occur throughout the Arctic

Sea, many have been reported only from the region of the great polar drift

from the Bering Strait westward to northeast Greenland, which accordingly has

a richer flora than is found elsewhere. Physical and chemical conditions,

particularly the latter, seem to be the main factors in determining the

distribution of diatoms, and the low and varying salinities of the surface

waters in the ice fields are not encountered in more temperate seas. It is,

therefore, not surprising that the diatoms found there are peculiar to that

habitat. Similar conditions are, however, encountered in the Antarctic but

the flora there is different, and very few species are common to the ice of

both polar regions.

       

DINOPHYCEAE

        The Dinophyceae, often referred to as the dinoflagellates or peridinians,

are a class of Algae which are generally unicellular, although a few filamen–

tous forms occur. The majority of them are found in the plankton, either of

the sea or of fresh waters, and they constitute one of the most important

elements of the phytoplankton. While typically they are pigmented forms with

holophytic nutrition, there are a considerable number which are colorless and

holozoic or parasitic.

        The unicellular forms are characterized by the presence of two flagella,

one of which is directed transversely and often encircles the cell more or

less completely, while the other is directed longitudinally. In most genera

these flagella are inserted on the side of the cell and lie in furrows, one

of which, containing the transverse flagellum, encircles the cell and is often

termed the girdle. The longitudinal flagellum is normally directed backward,

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and hence the longitudinal furrow is found on the part of the cell behind the

girdle, although it sometimes continues forward toward the apex. Both flagella

are used for progression, their undulating movements propelling the organism

forward with a rotating motion.

        The chromatophores of the colored forms are usually numerous and disk–

shaped, and dark yellow or brown owing to the presence of carotinoid pigments —

particularly the dark red peridinin, in addition to the green chlorophyll.

These chromatophores lie in the outer part of the cell where the protoplasm

is dense and granular. The inner part of the cell is occupied by the large

nucleus and by one or more vacuoles filled with a sap which is normally rose–

or calmon-colored. These vacuoles have a well-defined membran c e and definite,

often spherical, shape. Their function is probably excretory. The majority

of the genera possess of a cell wall consisting of cellulose, which often is

very complex and composed of many plates.

        The simplest and most primitive members of the Dinophyceae belong to the

subclass Desmokontae. In many of these the flagella are inserted at the apex

of the cell and there is no transverse, and frequently no longitudinal, furrow.

These are the only members of the class in which the longitudinal flagellum

is directed forward: they include some without a cellulose wall. In others

of this subclass the two flagella emerge from a single pore on the side of

the cell and both lie in furrows, the margins of which are often expanded

into wings. When these wings are well developed, the cell wall can have a

very complex shape, but it never consists of more than two portions joined

by a longitudinal suture.

        Reproduction is by longitudinal division and one half of the cell wall

goes to each daughter cell, which lays down a new second half. This subclass

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is represented in the marine phytoplankton of the Arctic by a number of species

of Exuviaella and Dinophysis , both genera with a cellulose cell wall. In

Exuviaella the flagella are apical, but in Dinophysis they are inserted

ventrally and lie in transverse and longitudinal furrows, respectively.

        The simples members of the remaining subclass, the Dinokontae, also

have no cellulose cell wall. In all of them, however, the flagella emerge

separately on the ventral side; and they all have well-marked transverse and

longitudinal furrows in which these flagella lie. Some of these forms have

chromatophores and are holophytic. Others are colorless and holozoic, ingest–

ing small particles of food by means of pseudopodia protruded from the antapical

part of the cell near the longitudinal furrow. Both holophytic and holozoic

species of this type are found in the phytoplankton of the Arctic, the genus

Gymnodinium being the best represented.

        Closely related to these are a number of forms which live as parasites,

mostly on various types of animals, of which planktonic copepods are the

favorites. In the parasitic stage they occur as unicellular cystlike

organisms either in the tissues of their host or externally, and show little

evidence of being Dinophyceae. Their affinities are revealed, however, by

the structure of the motile swarmers which they form from time to time. These

resemble the simpler members of the Dinokontae which have no cellulose cell

wall. One of this group occurs as a parasite on marine planktonic diatoms

of the genus Chaetoceros , and has been recorded from the waters around

Greenland.

        Most members of this subclass are forms with a definite cellulose cell

wall. This consists of a girdle forming the furrow in which the transverse

flagellum lies, and a series of unequal polygonal plates, the number and

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and arrangement of which differs from species to species. These plates have

knifelike margins and are firmly cemented together. They are usually pierced

by pores, which frequently lie in the center of the areolations with which

the plates of many species are ornamented. The shape of these armored

Dinokontae varies greatly. Peridinium, one of the commonest genera in the

sea and fresh waters in the Arctic, is subspherical, with an [ ?] indentation

in the region of the longitudinal furrow and sometimes two small horns or

spines at the antapical end. Goniaulax, another frequent arctic genus, is

very similar and only differs in the detailed arrangement of the plates. In

Ceratium , another common marine genus, there is one long apical horn an two

or three antapical horns of varying length, which may be bent sideways or

forward. The length of these horns in proportion to the size of the body

tends to be greater in tropical species of the genus than in those inhabiting

colder water, a fact shich is presumably correlated with the lesser density

of warmer water and the consequent need for more “form resistance” therein.

Almost all these armored forms are pigmented and holophytic.

        Reproduction in many species of Dinokontae is by binary fission during

movement. The plane of division is oblique and one flagellum goes to each

daughter cell, the other being regenerated. In some armored forms, including

many species of Ceratium , the cell wall ruptures along a lone between definite

plates. The exposed protoplast assumes the characteristic shape of the species

and the remaining plates of the cell wall are gradually developed. In others,

including Peridinium spp., there is no split but the new envelopes grow over

the surface of the daughter cells as they divide. In many other species

division occurs in a sedentary phase. The protoplast contrasts away from

the cell wall and divides obliquely in its contracted state. This division

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may occur before or after liberation from the old cell wall, which ruptures

to allow its contents to escape. In some cases the contracted protoplast

develops a new membrane before liberation from the parental one. The thin–

walled spherical cysts so formed may act as a resting stage of some duration,

but usually under favorable conditions they divide at once. These cysts

differ considerably from the thick-walled resting spores with ample food

reserves which are known in a number of armored freshwater species. These

are the means by which they survive unfavorable climatic periods. When they

germinate, the young individual which emerges from the ruptured cyst resembles

the unarmored members of the subclass, but in time grows a typical cellulose

envelope. Sexual reproduction in this group has never been observed, although

it is suspected to occur in some forms where unusually small unarmored swarmers

have been seen and interpreted as gametes.

        In addition to these flagellate unicellular forms, there are a number of

general of Dinophyceae which are normally nonmotile, existing either as single

cells or as short filaments. Most of these reproduce by the liberation of

biflagellate swarmers with one longitudinal and one transverse flagellum,

and with evident furrows. No such forms have, however, yet been recorded

from the Arctic.

        For a more detailed description of this class Fritsch (4) should be

consulted. Lindemann (11) also gives a fairly full account, together with

a synopsis of the genera and their characteristics, while Schiller (14)

gives descriptions and keys of all the species.

       

Arctic Occurrence

        The Dinophyceae are typically planktonic organisms. In the sea they

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play an especially important part in oceanic waters and in the warmer regions,

where they make up the bulk of the phytoplankton. In the Arctic they are

less important than the diatoms, but they nevertheless constitute an appre–

ciable part of the phytoplankton. In the spring, before the ice melts, some

of the naked forms have been found along with other flagellates constituting

a not very rich phytoplankton in the waters of the East Greenland current (2).

Such a community is probably widespread at that time of year, but further

observations are required before this can be established. This flagellate

community is replaced by diatoms when the ice melts, and these are often

found in great numbers. Their maximum is followed, when the water is warmer

but less rich in mineral salts, by a phytoplankton dominated by armored

Dinophyceae, members of the genera Ceratium and Peridinium being particularly

prominent (2; 7). The parasitic species Paulsenella chaetoceratis is also

found in the early summer plankton off Greenland on its host, the diatom

genus Chaetoceros. It is to be expected that the species parasitic on

zooplanktonic copepods, etc., will be found to be represented in the Arctic

also. The total number of marine species so far recorded from the Arctic

is about 200. More details of the part which the Dinophyceae play in

marine phytoplankton will be found in the article “phytoplankton.”

        About 10 species of Dinophyceae have so far been recorded from arctic

fresh waters (1; 15; 16), where they are apparently not a large component

of the algal flora. Little is known of the conditions which favor their

occurrence.

       

CHRYSOPHYCEAE

        This class of Algae consists mainly of unicellular and colonial forms.

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There are a few filamentous types known, but none of these have so far

been recorded from the Arctic. The chromatophore o s f this class are brown

or orange in color, owing to the presence of one or more accessory pigments.

They are normally large, few in number, and parietal in position. The

food reserves are stored as fat and as a substance known as “Leucosin”

which occurs as opaque rounded masses of a whitish color and is thought to

be a carbohydrate; starch does not occur. The protoplasm is always clear and

free from granules. Another characteristic feature of this class is the

formation of silicified cysts by its members. These cysts, which are formed

within the cell, are approximately spherical in shape, with various ornamenta–

tions on the surface, and with a pore closed by a cone-shaped, unsillicified

plug.

        There are a large number of motile unicellular flagellate members of this

class, but very few have been reported from the Arctic. In view of the fact

that in temperate regions the freshwater forms favor cold conditions, occurring

most abundantly in the winter and in cold mountain waters, this might seem

surprising were it not for the fact that most of them are so delicate as to

lose all their distinguishing features on preservation. They [ ?] therefore

need to be studied alive. Only two unicellular forms have so far been recorded

from the Arctic. One is a species of the often colonial genus Dinobryon and

will be described later. The other is chrysococcus rufescens , found by

Shirshov (15) in Novaya Zemlya. In this latter the cell is enclosed within

a rigid spherical envelope, leaving a pore through which the single flagellum

passes. In age, the envelope becomes colored brown by iron salts. Repro–

duction is by binary fission, one of the products escaping from the envelope

as a naked swarmer and forming a new individual, the other being retained in

the original envelope.

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        Two groups of unicellular members of this class are recorded from the

marine plankton of the Arctic. The Coccolithophoridadeae are represented

by about 10 species. In this family the cells have an outer membrane,

gelatinous at first if not later, in which a number of calcified inclusions,

the coccoliths, are deposited. These vary in shape from genus to genus but

are most frequently circular. They are often perforated in the center and

may bear processes on their outer sides which stick out from the cell like

stout spines. The gelatinous membrance of some species becomes calcified with

age, so that the coccoliths become embedded in a rigid envelope. In this

family there are normally two flagella and two thromatophores. In reproduction

the flagella are withdrawn, the protoplasm contracts somewhat, and then divides.

In some species the division in equal and both daughter protoplasts emerge from

the parental envelope as naked swarmers; in others it is unequal, the larger

protoplast remains within, and only the smaller one escapes.

        The Silicoflagellatae are only doubtfully referred to the class Chrysophyceae.

They possess an internal skeleton made of siliceous rods. Their chromatophores

are yellow or brownish-yellow, numerous, and discoid. Fat and leucosin are

both stated to occur as assimilatory storage products. There is a single apical

flagellum. Little is known of their life history, but reproduction appears to

occur normally by binary fission. Resting stages with no flagellum and a

distinct external membrane have been reported.

        Many of the flagellate Chrysophyceae are colonial, the cells being grouped

in various ways. Three general reported from Arctic fresh waters, Synura,

Chrysosphaerella , and Uroglena , have spherical colonies. In Synura and

Chrysosphaerella the cells are closely packed. In the latter of these they

are uniflagellate and on either side of the flagellum there is a small cup

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EA-PS. Ross: Algae: Planktonic Groups

from which a long, hollow, siliceous rod protrudes. There are also small

siliceous scales in the mucilaginous envelope of the colony. In Synura

the cells, each of which has two equal flagella, are united by the bases of

their membranes and there is no mucilage envelope for the whole colony. The

envelopes of the individual cells are tough and abovoid or ellipsoid in

shape. Uroglena has the cells at the surface of a large, roughly spherical

mass of mucus, and they are usually well separated. In all of these genera

the cells have two chromatophores. In none of them is the number of cells

in the colony definite, division of individual cells going on continuously.

Reproduction may occur by division of the whole colony or by the liberation

of swarmers.

        Dendroid as well as spherical colonies are found in this class of Algae.

In the genus Dinobryon the individual cells each possess an envelope with a

wide mouth and a pointed base. The two flagella of each cell are unequal.

Some species are solitary and one such has been recorded from the fresh waters

of Greenland. This reproduces by longitudinal fission, one of the daughter

cells escaping from the envelope and forming a new individual. In other

species, after longitudinal division one daughter cell migrates until it

is attached by its base close to the opening of the parental envelope. It

there lays down a new cellular envelope. Branched colonies are thus formed

which differ in shape according to the frequency and sequence of the divisions.

New colonies are formed by the liberation of swarmers, either after longitu–

dinal division of a cell or by the escape of its whole protoplast. In one

case fusion of swarmers formed by longitudinal division has been observed.

Spherical cysts with a short projection on one side are formed just outside

the cellular envelope in many species. The species concerned pass through

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EA-PS. Ross: Algae: Planktonic Groups

By means of these, the species concerned pass through unfavorable climatic

periods.

        Some members of this class are normally amoeboid in structure and holo–

zoic in nutrition, revealing their relationships only when they produce

swarmers of typical chrysophycean structure. None of these is recorded from

the Arctic. In other forms the normal state is a palmelloid one, concisting

of large numbers of rounded cells embedded in mucilage. The marine planktonic

Phaeocystis has large mucilaginous colonies with spherical lobes, in the surface

of which are embedded rounded cells with two yellowish [ ?] chromatophores.

This type reproduces both by detachment of lobes and by the liberation of

swarmers with two unequal flagella. Another palmelloid dorm is Hydrurus foetidus,

which occurs in running water and has been reported from Greenland. It forms

tufts of touch mucilaginous strands in which the cells are embedded. In each

branch there is an apical cell which divides repeatedly. The cells are

initially spherical in shape but become drawn out and pear-shaped with age.

Reproduction is normally by swarmers which are formed on short side branches

by the longitudinal division of a series of cells. In summer some cells are

protruded in mucilaginous projections from the surface of the thallus and these

form silicified cysts with a broad, delicate wing extending around half the

periphery.

        Fuller accounts of the morphology and life-history of this class of Algae

will be found in Pascher (12) and Fritsch (4).

       

Arctic Occurrence

        Both Coccolithophoridaceae and Silicoflagellatae are characteristic of

temperate and tropical oceanic water, and they are only found within the

Arctic where the currents bring in such waters. One silicoflagellate,

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EA-PS. Ross; Algae: Planktonic Groups

Distephanus speculum, is, however, comparatively abundant along the coast of

Greenland in the summer (1; 7). Phaeocystis pouchetii is found associated

with diatoms in their spring maximum towards the southern limits of the Arctic

area in the Atlantic (7), although it also is typically a temperate form.

Dinobryon pellucidum , on the other hand, is widely distributed in the Greenland

sea, Davis Strait, and Baffin Bay.

        The freshwater Dinobryon bavaricum var. vanhoeffenii was the dominant

member of the phytoplankton of the lake near Karajak Fjord in West Greenland

investigated by Vanhoeffen (16). Other freshwater members of this class have

been listed as occurring in various localities in Greenland (1), Novaya Zemlya,

and Franz Josef Land (15), and, though few in number of species, are apparently

often prominent members of the algal population.

       

XANTHOPHYCEAE

        This class of Algae, which is also known as the Heterokontae , is small

in numbers and sparsely represented in the Arctic by about 10 species, of

which one, Halosphaera viridis , is a marine planktonic form; the others

inhabit fresh water. Within the class there are to be found motile flagellate

forms, unicellular and colonial nonmotile forms, filamentous forms, and

siphoneous types. Within each type of group of forms, however, there is

little range of variation. The chromatophores are usually numerous and dis–

coid, and are of a yellowish-green color owing to the presence of an excess

of xanthophyll. Starch is absent, and oil is the usual food reserve. The

cell wall, when present, is rich in pectic substances and often consists of

two overlapping parts. The motile cells have two apically inserted flagella

of very unequal lengths. The longer of these is complex in structure, with

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EA-PS. Ross: Algae: Planktonic Groups

many fine side branches.

        In all the forms of this class so far reported from the Arctic, the

motile phase occurs only temporarily in reproduction. Halosphaera viridis

has large spherical cells, most of the interior of each being occupied by a

central vacuole. The chromatophores are numerous, flattened, and somewhat

angular. The pectic cell wall is slightly silicified and consists of two

equal halves joined at their margins. As the cells grows, the membran c e is

burst open and a new one is laid down. Reproduction is by swarmers with

two chloroplasts, and an oval or spherical shape. This species is common

in Gulf Stream waters and is found wherever these penetrate into the Arctic,

spreading farther as the summer advances. There is some indication that it

maintains itself throughout the year in the open water of the Barents Sea (18).

        Characiopsis is a unicellular epiphytic form from fresh water and has

been found in Franz Josef Land (15). The cell membrane is in two unequal

parts and is attached by a mucilaginous cushion at the foot of a stalk. The

mature cells are often multinucleate, and zoospores are formed by the proto–

plasm rounding off the smaller, upper part of the cell membrane.

        Ophiocytium recorded from Greenland (1) and Baffin Island (17), is

another genus which is primarily epiphytic. It is invariably multinucleate

and has the form of an elongate cylinder. The cell wall is of two unequal

parts of which the larger consists of a series of long thimble-like strata

with expanded margins. The other part fits over this like a lid, and becomes

detached when the reproductive bodies, which may be flagellate zoospores or

nonmotile aplanospores, are liberated. The three species recorded from the

Arctic are not normally epiphytic.

        Mischococcus confervicola , which has also been reported from Franz Josef

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EA-PS. Ross: Algae: Planktonic Groups

Land by Shirshov (15), is another epiphytic form. It consists of colonies

of rounded cells borne at the tips of more or less regularly forked mucilage

stalks. The cells contain two chromatophores. Reproduction is by unflagellate

motile spores which are formed singly or in pairs within the cells. Nonmotile

aplanospores are also frequently formed.

        In the genus Tribonema , which has been reported from Greenland (1), Novaya

Zemlya, and Franz Josef Land (15), the alga consists of unbranched filaments.

The cells are usually uninucleate and have a number of discoid parietal chromato–

phores. A sexual reproduction is by zoospores with two unequal flagella. These

are formed singly or in pairs in the cells. Nonmotile thick-walled aplanospores

are also known. These may form new filaments which they germinate, or they may

liberate one or two flagellate spores. Isogamous sexual reproduction has been

observed once. Occasionally Tribonema secretes irregular masses of mucilage

around its filaments and these become impregnated by ferric carbonate deposited

by bacteria which are thought to be symbiotic with the alga.

        Fuller accounts of the morphology and life history of this class will be

found in Fritsch (4) and Pascher (12).

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---

BIBLIOGRAPHY

1. Bachmann, H. “Beiträge zur Algenflora des Süsswassers von Westgrönland.”

Mitt.Naturf.Ges.Luzern , vol.8, pp.1-181, 1921.

2. Braarud, T. “The ‘Øst’ expedition to the Denmark Strait, 1929.” II. The

phytoplankton and its conditions of growth,” Hvalråd.Skr.

vol.10, pp.1-173, 1935.

3. Brown, R. “On the nature of the discolouration of the Arctic Seas,”

Bot.Soc.Edinb. Trans. vol.9, pp.244-52, 1868.

4. Fritsch, F.E. The Structure and Reproduction of the Algae . Cambridge,

England. The University Press, 1935. Vol.1.

5. Gran, H.H. Diatomaceae from the Ice-Floes and Plankton of the Arctic

Ocean. London, N.Y., Longmans, Green, 1904. Norwegian North

Polar Expedition, 1893-1896. Scientific Results no.11.

6. ----. “Die Diatomeen der Arktischen Meere,” Römer, and Schaudinn.

Fauna Arctica. Jena, 1904. Vol.3.

7. Grontved, J., and Seidenfaden, G. “The Phytoplankton of the waters west

of Greenland.” Medd.Grønland , vol.82, no.5, pp.1-380, 1938.

8. Heurck, Henri van. A Treatise on the Diatomaceae . Trans. by W.E. Baxter.

London, Wesley, 1896.

9. Hustedt, F. “Diatomeen aus der Umbegung von Abisko in Schwedisch-Lappland,”

Archiv für Hydrobiol. Vol.39, pp.82-174, 1942.

10. Karsten, Gustav. “Diatomeae,” Engler, Adolf, and Prantl, K. Die

Natürlichen Pflanzenfamilien . Leipzig, 1928. Vol.2.

11. Lindemann, E. “Peridineae (Dinoflagellatae),” Naturl. pflFam., 2 aufl.,

vol.2, pp.3-104, 1928.

12. Pascher, A. “Heterokonten,” Rabenhorst’s Krypt. -Flora Deutschl . Vol.11,

pp.1-1092, 1937-9.

13. Ross, Robert. “Freshwater Diatomeae,” Polunin, Nicholas, ed. Botany

of the Canadian Eastern Arctic, Pt.II, Thallophyta and

Bryophyta . Ottawa, 1947. Nat.Mus.Can. Bull. no.97.

Biological Ser. no.26.

14. Schiller, J. “Dinoflagellatae (Peridineae) in monographischer Behandlung”

Ibid. vol.10, no.3, pp.1-617; 1-589, 1931-7.

024      |      Vol_V-0092                                                                                                                  
EA-PS. Ross: Algae: Planktoni g c Groups - Bibliography

15. Shirshov, P.P. “Ecologic-geographical essay on the fresh-water algae

of Novaya Zemlya and Franz Joseph Land,” Leningrad.

Arkticheskii Nauchn. - Issled. Inst. Trudy , no.14, pp.73-162,

1935.

16. Fanhöffen, E. “Die Fauna and Flora Grönlands,” Grönland-Exped. Gas.E r R dk.

Berlin , vol.2, pp.1-383, 1897.

17. Whelden, R.M. “Algae,” Polunin, N. Botany of the Canadian Eastern Arctic .

Part II, Thallophyta and Bryphyta , Ottawa, 1947. Nat.Mus.Can.

Bull . no.97. Biological Series no.26.

18. Wulff, A. “Ergebnisse der Untersuchung des Oberflächenplanktons,” Ber.Wiss.

Komm.Meeresforsch. n.f., vol.4, pp.313-37, 1929.

       

R. Ross